Hydraulic fluid circuits use a host of fluid powered components to perform work within a system. Pressure within the fluid circuit is provided via a fluid pump. Supplemental fluid pressure may be provided by a hydraulic accumulator. The accumulator acts as a standby energy storage device, typically in the form of a pressure storage reservoir. Hydro-pneumatic accumulators in particular are charged when fluid pressure in the circuit moves a piston within a cylinder. Movement of the piston compresses a volume of inert gas within the cylinder, thus allowing oil to flow into the accumulator.
When a fluid pump is turned on, fluid is delivered under pressure to all open branches of the fluid circuit. The accumulator piston begins to compress the charging gas once the various fluid passages of the circuit have completely filled with oil. When the fluid pressure in the circuit overcomes drag of the piston seal on the cylinder wall, the piston will begin to move in its compression stroke. Just before such movement begins, the gas pressure in the accumulator is at equilibrium with fluid pressure in the circuit. This pressure point is known as the accumulator pre-charge pressure.